Syrian government forces and allied militias have made a swift advance into rebel-held suburbs of Damascus, threatening to disrupt a years-long stalemate in the war around the Syrian capital.

The development came as the UN envoy for the war-torn country vowed in Geneva that peace talks "are not going to be abandoned, frankly, ever".

Staffan de Mistura said he is waiting to see "gestures" following a Vienna meeting of world and regional powers earlier this week, before setting a target date for the resumption of UN-mediated indirect talks between Syrian president Bashar Assad's representatives and opposition groups.

Fighting in Syria continued in earnest on Thursday as air and missile strikes pounded rebel positions in Damascus's eastern Ghouta suburb, clearing the way for soldiers and Hezbollah militants to seize valuable farmland that nourished residents trapped in the blockaded area, according to two activist groups.

An opposition media activist inside the Ghouta enclave said news of the advance is a blow to the opposition. "It is agricultural land," said the activist.

Opposition groups expelled government forces from the Ghouta region, which hugs Damascus from the east and south, in 2012, a pivotal development in the conflict that began as a popular uprising demanding government reforms the year before.

Syrian forces, supported by Lebanese and Iranian fighters and Russian air power, responded by encircling the zone and slowly tightening its blockade. Residents and rebels inside have depended on smuggling routes and local farmland to survive.

Syrian state media, which also reported the advance, said the army took advantage of rebel infighting in the eastern Ghouta pocket to make its strike. Around 500 militants have been killed in three weeks of infighting, according to the Britain-based Syrian Observatory for Human Rights.

The rebel ultra-conservative Islam Army has dominated the enclave since its fighters expelled Islamic State militants in 2014, but has drawn the ire of other rebel factions and civil society groups for its high-handed rule.

The militia's forceful leader Zahran Alloush, died in an air strike on Christmas Day, 2015. Tensions exploded into open warfare last month as moderate rebels and al-Qaida-linked extremists arrayed against the Islam Army.

"Regrettably, these factions aimed their weapons at each other instead of the regime," said the Ghouta activist. Islam Alloush, spokesman for the Islam Army, blamed competing factions for restricting the militia's movements. "It was a natural outcome," he said of the government advance.

Thursday's advance has displaced hundreds of families, the Observatory said. A Syrian advocacy group, the Syria Campaign, said the estimated number of families that have fled the southern parts of East Ghouta in the past couple of days is about 2,400 families, including more than 500 families that fled Thursday morning.

The group said that scores of families took refuge in other nearby besieged towns, with no appropriate shelter or protection.

The Lebanese Shiite militant Hezbollah group, which like Iran is a staunch supporter of Assad, has lost over 1,000 fighters in Syria, including many of its top leaders.

Hezbollah has blamed rebels for artillery shelling that killed its top military commander Mustafa Badreddine last week. He was killed near the Damascus airport, close to eastern Ghouta. Rebel groups have not claimed responsibility for the shelling and the Observatory has denied there was shelling at the time in the airport's vicinity.

Also on Thursday, activists said suspected government war planes carried out air strikes in the rural part of the central Homs province, killing at least 11 civilians including at least three children.